This invention relates to a roof bolt assembly including mechanical point anchors, such as disclosed in U.S. Pat. No. 4,410,296 to Unrug, issued Oct. 18, 1983, for use underground, particularly in the roof of a coal mine.
In spite of the large technological advancements in mining and introduction of extensive safety regulations, roof falls are still the largest single cause of fatalities in coal mines. The statistics show, however, that less than one percent of roof falls are responsible for personal injury, but large numbers of falls occur in practically every mine. There is also a substantial cost involved in cleaning of the roof falls, reaching about twenty million dollars a year in Kentucky alone.
Roof conditions depend on the geological composition of the roof strata; strength of pillars and floor; and geometry of the mine opening. The geometry of openings can be influenced by design; however, only within certain limits imposed by the size of mining machinery and overall requirements of mining technology.
The geological composition of a roof is the major controlling factor of the roof stability. It depends on the rock creating an immediate roof at the opening. There is an established view that shaly roofs are potentially unstable, while sandstone roofs do not create problems.
The major factor influencing the rate of deterioration of the shaley mine roofs is a time-dependent weathering process. Shales are fine grain marine deposits with weakness planes perpendicular to the direction of the sedimentation. The tensile strength across those planes is much smaller than in a material without such planes. It has been found through extensive laboratory and field research that combinations of changes in temperature and humidity contribute to the shrinking and expansion of the shale material, as well as opening of the cracks along the weakness planes, where condensed moisture penetrates into the rock by diffusion and capilar pressure. Then the total area being influenced by moisture and its changes increases, as well as the total volume of affected rock. The deterioration process goes on and finally the roof begins to spoil, primarily between the bolts. Later, scalling shale falls from under the roof plates and the bolts lose their tension.
The above-described causes of deterioration take into account only roof surface as an exposed part of the rock. In reality, roof bolts are installed in holes drilled in the roof of a depth required by the roof plan for given conditions. As a consequence, the annular space between the bolt rod and roof rock becomes filled with mine air. Thus, the changes in humidity and temperature affect the side of the holes in the roof. These holes are normally drilled in a pattern three to four feet apart from each other. Those holes extend through the bedding and other weakness planes in the roof. Consequently, not only surface area of the visible roof is exposed to the influence of weathering, but also an invisible part of the roof rock in the vicinity of each hole.
A roof bolt with a mechanical bolt anchor--expansive shell such as disclosed in U.S. Pat. No. 4,410,296 to Unrug, works on the principle of an activated wedge, which is pulled downwards by rotation of the threaded end of the bolt rod which is threaded into the anchor. The wedge movement exerts a pressure on the expansive shells which press against the rock, sides of the roof holes. The shells have teeth-like protrusions to prevent slippage of the bolt anchor. The typical installation torque is 150 lb, which assures pretensioning of the roof bolt and compression of roof strata between the anchor and bolt plate against the roof surface.
From a rock mechanics standpoint, the pretensioning a stratified, shaley roof is essential, because it increases friction in a contact plane of strata and prevents separation. The mechanical principle of a bolted roof is a so-called "clamped beam effect" where strata are compressed and forced to work together. In this way, they are much stronger than when they deform one after one. The analogy exists between the mine roof and a car spring, where is order to have a comfortable ride in a car, provisions are made to allow particular leaf springs to slide on each other, thus assuring smooth bending of the whole package. The mine roof support is based on this same principle, but used for the opposite purpose, i.e., to increase stiffness by providing an additional friction between the roof members.
From the above explained principles of how a roof bolt works, it is clearly apparent that a decrease of bolt tension is detrimental for the mine roof. It can dramatically speed up the process of roof deterioration, and finally cause a roof fall.
In the contact area of the anchor shells against the rock hole sidewalls, large stresses exist, sometimes exceeding rock strength and thus producing fractures. Those fractures can also be affected by the moisture condensation and drying of rocks in the opposite cycle of the mine air seasonal changes. In other words, the destruction of the rock in the most strained part interacting with the shell can be accelerated, resulting in the bleeding of tension on a bolt.
Finally, the steel used for roof bolt is an inexpensive one which corrodes when exposed to moisture. It is well known that when humidity and temperature change, there is a condensation of moisture on metal objects in the mine. This can be seen when removing bolts from the mine roof. Rust at the rod does not greatly influence the performance of the bolt when superficial. But it is more critical on threads where fine dimensions and high stresses cause corrosion, which again contributes to the bleeding of tension.
The changes in mine atmosphere are caused by seasonal weather changes and cannot be controlled in a feasible way. During the winter cold air entering the mine (due to the mine ventilation) heats up by contact with the mine being warmer. So the air is able to absorb the moisture and it dries up the mine. During the summertime, the opposite process takes place. Warm and humid air enters the mine and cools off, thus causing condensation moisture, which is absorbed by the mine. Often visible drops of condensed water hang from the roof and especially metal parts of bolts.
Accordingly, a need in the art exists for a sealed roof bolt assembly to eliminate the exchange of air in the roof bolt holes by sealing entrances of the holes.